1. Field of the Invention
The present invention relates to inter-vehicle distance estimating method and apparatus for an automotive vehicle which estimate an inter-vehicle distance from the vehicle to a preceding vehicle which is running ahead of the vehicle, for example, using both of a radar and a camera mounted on the vehicle, from an image trapped by the camera in order to effect an automatic braking for the vehicle.
2. Description of the Related Art
A U.S. Pat. No. 4,987,357 issued on Jan. 22, 1991 exemplifies a first previously proposed inter-vehicle distance estimating apparatus applied to an adaptive motor vehicle cruise control system in which a video image of the preceding vehicle located in the forwarding direction of the vehicle is stored, this image is processed under a scaling, this image including that of the preceding vehicle is trapped, and the inter-vehicle distance to the preceding vehicle is estimated from a displacement of the preceding vehicle which occurs during a trapping of the image of the preceding vehicle.
A second previously proposed inter-vehicle distance estimating apparatus is exemplified by a Japanese Patent Application First Publication No. Heisei 10-143799 published on May 29, 1998.
The video image including a part of the preceding vehicle is stored as a template, a present image position of the preceding vehicle is approximately predicted according to an achievement of several times scalings, and the template is processed under another scaling so as to be coincident with a window clipped at the image position.
A third previously proposed inter-vehicle distance estimating apparatus is exemplified by a Japanese Patent Application First Publication No. Heisei 10-97699 published on Apr. 14, 1998.
In the third previously proposed inter-vehicle distance estimating apparatus, presuming that the preceding vehicle (obstacle) is present within a region enclosed by vertical and horizontal edges, the inter-vehicle distance from the region to the preceding vehicle is estimated.
Furthermore, an inter-vehicle distance setting apparatus described in the Japanese Patent Application Second Publication No. Heisei 11-44533 published on Feb. 16, 1999 stores the video image including a part of the preceding vehicle as the template while the inter-vehicle to the preceding vehicle is being detected by a laser radar, derives an image region having a most similarity to the template through a correlation process, and estimates a spatial distance to the position of a center of the image region as the inter-vehicle distance.
It is noted that such a kind of inter-vehicle distance estimating apparatuses as a combination of both of the camera and the radar estimates the inter-vehicle distance from the image photographed by the camera to the preceding vehicle due to the following reason. That is to say, for example, there is a limitation in a range in which a laser light beam or millimeter-wave is propagated in order to detect the inter-vehicle distance up to a relatively long distance (up to approximately 100 meters) and, on the contrary, a range within which the inter-vehicle distance can be detected (10 meters or shorter) becomes narrower.
On the other hand, although a microwave radar or an ultrasonic sensor has a relatively wide detection range even in a proximity to the vehicle, the distance detectable thereby is short. Hence, the inter-vehicle distance cannot be detected for the preceding vehicle which is located at a position far away from the vehicle on which the microwave radar or ultrasonic sensor is mounted. In order to compensate for such disadvantage as described above, the inter-vehicle distance is, thus, estimated from the image photographed by the camera to the preceding vehicle.